1. Field of the Invention
The present invention concerns the field of ophthalmic lenses.
It concerns, more particularly, contact lenses and intra-ocular implants. Unifocal contact lenses are designed to be placed on the cornea of the eye, and they are used to compensate for ametropias (myopia or hypertropia). By contrast, the intra-ocular implants are designed to replace the crystalline lens inside the eye.
2. Description of the Prior Art
It has been sought, for many years, to develop bifocal or multifocal contact lenses placed before the cornea, enabling compensation for the accommodation of presbyopic patients and giving the possibility of correcting long-sightedness and, at the same time, assisting the crystalline lens in close vision.
Many types of bifocal or multifocal contact lenses have already been proposed. However, none of these multiple focal lenses, proposed until now, have been entirely satisfactory.
As illustrated schematically in the appended FIGS. 1A and 1B, respectively corresponding to a front view and a lateral sectional view, the document FR-A-1 423 908 refers to a contact lens 10, formed by two regions 11, 12, possessing, on at least one face, different radii of curvature and, hence, different refractive properties, respectively adapted to close vision and far or distant vision. The use of the lens described in the document FR-A-1 423 requires a relative shift between the lens and the eye at the change from close vision to far vision, through the resting of the lens on the lower eyelid, so that the axis of vision O--O passes alternately through either of the two regions 11, 12. The appended FIGS. 2A and 2B schematically illustrate the theoretical, relative position of the lens 10 and the eye, respectively in far vision and close vision. In these FIGS. 2A and 2B, a distant object is referenced 13, a close object is referenced 14, while the retina, the natural crystalline lens and the lower eyelid are referenced 15, 16 and 17 respectively. A lens of this type does not give full satisfaction. It will be noted, in particular, that this type of lens theoretically requires a major shift of the lens 10 with respect to the eye so that the axis of vision O--O moves alternately through either of the two regions 11, 12. Consequently, the indispensable resting of the lens 10 on the lower eyelid 17, combined with the relative lens/eye shift, make these lenses hard to bear. If, moreover, both types of correction are used simultaneously, the optical axis O--O being close to the junction between the two regions 11, 12, an unacceptable jump in vision is obtained.
It will be noted that, as shown in FIGS. 3A, 3B and 4A, 4B, respectively in front view and in a lateral sectional view, the documents U.S. Pat. Nos. 1,647,721 and 1,735,758 refer to lenses 20, 30 designed to be mounted on spectacle frames which have, similarly to the document FR-A-1 423 908, regions 21, 22 and 31, 32 having different refraction properties; however, according to the documents U.S. Pat. Nos. 1,647,721 and 1,735,758, the different refraction properties are not obtained by variation in radius of curvature, but by variation in index. More precisely, the different refraction properties are obtained by inclusion, in the body 22, 32 of the lens formed by a material with a determined index, of an element 21, 31 formed by a material with a different index. The lenses defined in the documents U.S. Pat. Nos. 1,647,721 and 1,735,758 have the same drawbacks as the lens shown in the document FR-A-1 423 908.
The prior art contact lenses, which have been described briefly above with reference to FIGS. 1 to 4, are designed to enable an alternating vision. These lenses generally make it necessary to have a ballast to keep their orientation and prevent their rotation on the eye, a ballast that forms an excess thickness which is highly irksome with respect to the wearer's comfort.
Moreover, the shift of the lens should be perfectly controlled to ensure accurate alternating vision. Now, because of different variations, which are independent of the geometry of the lens, such as palpebral pressure, the flow of the tear fluid film, etc, this shift may become uneven and uncontrolled.
To overcome these drawbacks, other contact lenses have been proposed and use the concept of simultaneous distant and close vision.
As illustrated schematically in the appended FIGS. 5A and 5B, respectively showing a front view and a lateral sectional view, the documents EP-A-1 184 490, EP-A-0 232 191, U.S. Pat. No. 4,636,049 refer to a contact lens 40, formed by two concentric regions 41, 42 having, on at least one face, different radii of curvature and, hence, different refraction properties, respectively adapted to close vision and far vision. These lenses no longer require any relative shift between the lens and the eye to go from one type of vision to another. However, a superimposition of images is observed at the transition between the two regions. Moreover, the component is very sensitive to variations in the pupil diameter and this pupil may undergo great variations in diameter depending on the luminance and on the patient. For example, if the central region 41 of the lens is assigned to close vision, when the aperture of the lens is such that it covers only the central region 41, the user cannot see a distant object properly.
The document U.S. Pat. No. 3,726,578 refers to a lens which has, similarly to the documents EP-A-0 184 490, EP-A-0 232 191 and U.S. Pat. No. 4,636,049, concentric regions having different refraction properties. However, according to the document U.S. Pat. No. 3,726,578, the different refraction properties are not obtained by variation in radius of curvature but by variation in index, namely by inclusion, in the body of the lens formed by a material of a determined lens, of an element formed by a material of a different index.
The document U.S. Pat. No. 3,339,997 refers to a lens very similar to the foregoing, but one which, using the chromatic aberration of the eye, has two concentric or non-concentric regions, with different chromatism in order to have different properties of refraction depending on the wavelength.
The documents U.S. Pat. Nos. 3,004,470, 4,162,122, 4,210,391 and 4,340,283 refer to lenses 50, 60, schematically illustrated in a front view in FIGS. 6B and 6C, no longer having only two concentric regions as advocated by the documents EP-A-0 184 490, EP-A-0 232 191 and U.S. Pat. No. 4,636,049, but a series of ring-shaped zones 51, 52; 61, 62, alternately having radii of curvature with a first value and a second value 53, 54, 63, 64 to work alternately in close vision and far vision. The alternation of zones makes it possible to do away with the problem of sensitivity to the variation of the pupil diameter (this is a definite advantage with respect to the object of the documents EP-A-0 184 490, EP-A-0 232 191 and U.S. Pat. No. 4,636,049). However, the lenses referred to in the documents U.S. Pat. Nos. 3,004,470, 4,162,122, 4,210,391 and 4,340,283 make no mention of the diffractive properties of alternating contours of this type.
The documents EP-A-0 064 812 and U.S. Pat. No. 4,637,697 refer to another type of lens, designed to achieve a bifocal correction of the eye with zone separation. For this, the lenses proposed in the documents EP-A-0 064 812 and U.S. Pat. No. 4,637,697 have a rear surface and a front surface with mean curvatures adapted to the correction needed for the distant vision of the wearer, the lens further having a hologram which gives the lens an additional diffraction power, such that the image of a near object is properly focused on the retina. According to the above-mentioned documents, the hologram may be generated within the lens or on its surface. The hologram may be generated by holographic recording. It can be generated mechanically in the form of a hologram in relief on the surface of the lens, as shown in the appended FIG. 7, i.e. in the form of a zoned system inspired by the optical system known to those skilled in the art as the SORET system.
More precisely again, the holograms in relief according to the document U.S. Pat. No. 4,637,697 are formed by concentric zones 70, 71, 72, 73, 74 of a same area, i.e. the external radii of which develop according to a geometric progression in .sqroot.Kr.sub.1 where r.sub.1 designates the external radius of the central zone 70, and K designates whole numbers. The phase contour of the hologram in relief is identical for all the zones 70, 71, 72, 73, 74 and is asymmetrical to further +1 order diffraction. By way of example, the phase contour of each zone 70, 71, 72, 73, 74 may be defined by a sequence of A concentric levels with different thicknesses to obtain differences in optical retardation of 2.pi./A. According to the document EP-A-0 064 812, the hologram may be designed to be on the entire optical zone of the lens or on only a part of it. Moreover, the lens can be made with several holograms generated separately, superimposed if necessary, giving different diffraction power values. In the case of holograms in relief forming a zoned system, the obtaining of different diffraction power values takes the material form of zones consisting of concentric sub-zones, the external radii of which respond to geometric progression values that differ from one zone to the other. The use of lenses of the type referred to in the documents EP-0 064 812 and U.S. Pat. No. 4,637,697 raises problems of perception of contrasts under certain conditions of vision.
It will be noted that the document GB-A-802 918 previously described the association, in a sighting system for firearms, firstly of optical means enabling the user to see a distant target clearly and, secondly, a zoned system enabling the user to observe the fore-sight of the weapon. According to the document GB-A-802 918, the zoned system may be formed either by a transparent lens, having one of its faces formed by alternately transparent and opaque, concentric zones with the same area, or by a transparent lens formed by concentric rings of the same radial area but having, alternately, either of two optic thicknesses.